The present invention relates to an image forming apparatus such as an electric-photo printer, a copier, and a facsimile. In particular, the present invention relates to a fixing device of the image forming apparatus.
In a conventional image forming apparatus such as an electric-photo printer, a copier, and a facsimile, a fixing device heats developer transferred to a recording medium to melt, and presses the developer with a specific pressure to fix the same on the recording medium (refer to Patent Reference 1). The fixing device is provided with a heat source for supplying heat necessary for the fixing process. The heat source is controlled to have a temperature within a specific temperature range at which the fixing device can fix the developer.
Within the specific temperature range, developer or toner suitable for the recording medium can be properly fixed. At a temperature below the specific temperature range, it is difficult to adequately melt the toner. As a result, the toner is stuck to the recording medium with a strength less than a desirable level, and the toner may be stuck to a fixing roller, not the recording medium, a phenomenon called cold off-set. To the contrary, when a temperature exceeds the specific temperature range, the toner has a viscosity lower than a desirable level. As a result, the toner tends to adhere to the fixing roller, not the recording medium, a phenomenon called hot off-set.
The specific temperature range depends on a type of recording medium or a thickness thereof, and is typically stored as a table of set temperatures corresponding to a type of recording medium and a thickness thereof.
When an image forming apparatus starts printing, a temperature sensor disposed on a fixing device detects a temperature of the fixing device, so that it is determined that the fixing device is within a specific printable temperature range. When the temperature of the fixing device is below the specific printable temperature range, a target temperature suitable for a recording medium to be printed is set. Then, a heater of the fixing device is turned on. After the temperature of the fixing device becomes within the specific printable temperature range, a target temperature suitable for a recording medium to be printed is set without waiting for the fixing device to warm up. Then, the heater of the fixing device is turned on, and the image forming apparatus starts printing.
When the temperature of the fixing device is above the specific printable temperature range, a target temperature suitable for a recording medium to be printed is set without turning on the heater. After the fixing device is cooled down to the specific printable temperature range, the image forming apparatus starts printing.
Patent Reference 1: Japanese Patent Publication No. 10-104990
A target temperature is set, in general, such that the fixing device is heated up to a high temperature as possible, since it is sometimes difficult to predict the number of media. Accordingly, it is possible to prevent the fixing device from cooling down below the specific printable temperature range when a large number of media pass through the fixing device. However, when a target temperature is set such that the fixing device is heated up at a high temperature as possible, and only a small number of media pass through the fixing device, the fixing device is not cooled down. In an extreme case, the fixing device may be heated up above the specific printable temperature range, a phenomenon called over-shoot.
In this case, after a heater and a fixing motor are turned on, the fixing device fixes toner on a recording medium. Then, the recording medium is discharged, and the heater and the fixing motor are turned off. At this moment, the fixing device may be suddenly heated up above the specific printable temperature range, thereby causing the over-shoot. Once the over-shoot occurs, it is necessary to wait until the fixing device is cooled down into the specific printable temperature range.
On the other hand, when too many media are printed, the fixing device is cooled down below the specific printable temperature range, a phenomenon called under-shoot. Once the under-shoot occurs, it is necessary to wait until the fixing device is heated into the specific printable temperature range.
In view of the problems described above, an object of the present invention is to provide an image forming apparatus, in which it is possible to prevent the over-shoot or under-shoot from occurring.
Further objects and advantages of the invention will be apparent from the following description of the invention.